The effect of screening on road surface temperatures is analysed using data from the Swedish road weather information system (RWIS) together with data from thermal mapping. The study deals with the analysis of temperature variations caused by shading of the road surface during clear daytime conditions and focuses on the relation between solar elevation and magnitude of road surface temperature differences between screened and sun-exposed areas. Also included is an analysis of how the temperature differences during the day affect the establishment of temperature variations after sunset and the time it takes after sunset for temperature differences caused by shading to decline. The results show that the magnitude of road surface temperature differences between sun exposed and screened sites (RSTdiff) that develop during clear day conditions can be attributed to solar elevation (β) and can be expressed by the equation: RSTdiff=−2.7+0.46(β). A relationship between the daily maximum temperature difference andthe preservation of the screening effect after sunset is observed. The effect after sunset can be described by: RSTdiff=A−Bh+Ch2−Dh3−Eh4+Fh5, where h is the time in hours after sunset and A to F are constants related to the time of the year at the actual site.

The Otaki Precipitation Estimation by Radar (OPERA) programme was designed to investigate the processes that lead to enhancement of rainfall over the Tararua ranges of New Zealand. These ranges rise to 1500 m above the coastal plain and enhancement of rainfall by windflow over these hills leads to annual hill-top rainfall of over four times that upwind. The OPERA experimental campaigns aimed to characterise the enhancement processes by analysing data collected from a transect of high-resolution rain gauges and a locally deployed, high-resolution radar, supported by scanning radar and satellite observations. Measurements made during these experiments showed that orographic enhancement led to hill-top accumulations often twice that upwind, and up to as much as a factor of seven in one case. The data suggest that the most frequent occurring enhancement mechanism was triggered convection. This mechanism leads to an increase in rainfall over the hills of around a factor of two, primarily through an increase inthe duration of rain. Seeder/feeder-type enhancement occurs less frequently but leads to larger enhancements.

Hailfall is one of the most dangerous and damaging phenomena associated with severe thunderstorms. Its remote identification is still under investigation, especially by means of polarimetric radar data. The need to use polarimetric measurements is a limit to operational implementation of the techniques developed up till now. The routine sampling of radar allows the computation of the differential hail signal HDR (Aydin et al., 1986) every 15 minutes. The assessment of the HDR is performed by comparison with the ground truth from a hailpad network. The spatial resolution of this network is quite coarse (one pad per 16 km2) for the collection of all the hail shafts, as they generally have a small horizontal extent and are short lived. Such bounds have been considered in the analysis of four large hailfalls recorded in 1997. This study reveals also the limitation on radar hail detection arising from the frequent occurrence of strong differential attenuation. Nevertheless, to identify regions where a sharp rain-hail transition occurs, polarimetric measurements work much better than algorithms based only on horizontal reflectivity. Results show that use of HDR gives a high probability of correct hail detection as well as an acceptable number of false alarms.

A cold-air comma cloud crossed southern Britain early on 18 February 1997 bringing strong gusts and convective precipitation. Development of the comma cloud occurred in association with a positive upper tropospheric potential vorticity (PV) anomaly. A mesoscale analysis of the comma cloud was carried out using special three-hourly soundings from UK radiosonde stations together with data from rainfall radar, satellite, NWP model output and a wind-profiling radar. The analysis reveals that the comma cloud tail was of ana-form whilst the polar cold front (behind which the comma cloud developed) was kata-form. Further analysis indicates that the polar cold ?w front and the comma cloud front were tending to become one entity at the surface and can thus be considered an example of a kata-cold front undergoing a partial transition to more of an ana-form locally in the region of an upper tropospheric positive PV anomaly.

Errors and uncertainty in radar estimates of precipitation result both from errors in the basic measurement of reflectivity and from attempts to relate this to the precipitation falling at the ground. If radar data are to be used to their full potential, it is essential that effective measures are taken to mitigate these problems. The automatic processing of radar data that forms part of the UK Met. Office's Nimrod system addresses a number of specific sources of error. These include the identification and removal of spurious echoes resulting from anomalous propagation of the radar beam, errors resulting from variations in the vertical profile of reflectivity and radar sensitivity errors. Routine verification of the surface precipitation estimates has been undertaken, largely through comparison with rain gauge observations, over a range of timescales, which has allowed the benefits of the quality control and correction processes to be quantified. Although the improvement derived varies according to the dominant synoptic situation, an average reduction in the root-mean-square difference between gauge and radar data of 30% can be achieved.

We study the contribution of surface data to convection nowcasting over regions of modest orography and under weak synoptic forcing. Hourly mesoscale analyses are performed using the CANARI optimal interpolation analysis scheme, which combines first-guess fields from the fine mesh (10 km) ALADIN model with hourly routine observations arising from a mesonet of automated ground stations. These analyses then allow the computation of diagnostic parameters that quantify convective instability, low-level lifting processes and moisture supply: these are the convective available potential energy (CAPE) and the moisture convergence (MOCON). A tuning of the analysis scheme is needed first for it to fit the meso-?-scale. Then, the skill of the computed diagnostics for convection nowcasting is evaluated by comparing their fields with radar reflectivities observed between one and four hours after the analysis time. This is done for four selected convective situations. With regard to thunderstorm triggering, results show that this usually happens over areas of persistently high values of CAPE which undergo convergence continuously from four to one hour before the event; on the other hand, areas of persistent divergence are never associated with convective developments. In addition, the proposed criteria allow a significant reduction in the areal extent of predicted thunderstorms (i.e. decreasing the false-alarm rate) compared with what can be currently done on an operational basis, while maintaining a low non-detection rate. As to convection monitoring, we find that the organization of convective systems into a reflectivity line is preceded by a similar organization in the MOCON field from one to three hours ahead.

The two latest changes introduced during 1998 into the Ensemble Prediction System (EPS) operational at the European Centre for Medium-Range Weather Forecasts (ECMWF) are described. The first change, the inclusion of instabilities growing during the data assimilation period in the generation of the EPS initial perturbations, increased the probability that the analysis lies inside the ensemble forecast range. The second change, the introduction of a simulation of random model errors due to parametrized physical processes, improved in particular the prediction of precipitation. The performance of the ECMWF Ensemble Prediction System from 1 May 1994 to 2 March 1999 is assessed using different statistical measures. Results indicate that the general performance of the EPS has been improving over the years. Finally, ongoing research projects on predictability issues developed either at ECMWF or at European research institutes in collaboration with ECMWF are discussed

A mesoscale model has been applied over the Greater Sydney region to an air pollution episode resulting from fire hazard reduction burns from 12 to 14 April 1997. The episode was characterised by two distinct events in a period of sustained light winds, which included humid sea breezes and land breezes. The first event occurred early in the period when the atmospheric circulation involved either weak sea breezes or no sea breeze at the coast. Described here are the meteorological conditions associated with the inter-regional transport of smoke from a prescribed, or hazard reduction, burn just north of Sydney, which concentrated smoke in the eastern part of the Sydney metropolitan area. The second event, which occurred late in the episode, was local in nature but was induced synoptically by the large-scale airflow by a change in wind direction that transported smoke and fog over a major highway. Also described are the meteorological conditions that contributed to the disruption of a major highway caused by a multiple vehicle pile-up just to the south-west of Sydney later in the period. Numerical model simulations using the University of New South Wales (UNSW) NWP model, which used archived real-time data, accurately predicted both the concentration of pollution in the eastern part of the Sydney metropolitan area in the first event and the hazardous smog formation and timing of the clearing south-west wind change in the second event.

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